Automatic adjusting arrangement for electrical networks



Sept. 11, 1945. A. c, CORNER 2,384,727

AUTOMATIC AI LTUSTING ARRANGEMENT FOR ELECTRICAL NETWOR KS 4 Sheets-Sheet 1 Filed Aug. 10, 1943 IP1 T1 1P2 i ICA AGC

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A C Carver :l FrFEMEg EMMEELCL h Efimg. EP MEEQ Sept; 11,1945. A. c. CORNER AUTOMATIC ADJUSTING ARRANGEMENT FOR ELECTRICAL NETWORKS Filed,Aug. 10, 1943 4 SheetsSheet 3 1. 5 Z W m ag iw w VF m EE EE g F ECMFE E M A. c. CORNER 2,384,727 AUTOMATIC ADJUSTING ARRANGEMENT FOR ELECTRICAL NETWORKS Sept. 11, 1945.

4 Sheets-Sheet 4 Filed Aug. 10, 1943 H HU h H H mi m W? 5 1 A 5% m MW y K 2 H H m m 5 m m? D 1 m G Mm an m d EEMQE E W E COK 1 Patented Sept. 11, 1945 AUTOMATIC ADJUSTING ARRANGElWENT FOR ELECTRICAL NETWORKS Arthur Charles Corner, Calcutta, India, assignor to Automatic Telephone & Electric Company Limited, London, England, a British company Application August 10, 1943, Serial No. 498,126

In Great Britain August 18, 194.2

8 Claims. (01. 178-44) The present invention relates to automatic adjusting arrangements for electrical networks and finds convenient application to the problem of securing automatic gain control in carrier telecommunication systems. The main object of the invention is to provide a simple and reliable adjusting device preferably made up of components which are already in successful use in other electrical control systems such as automatic telephone systems.

Automatic gain control arrangements as hitherto employed generally make use of a potentiometer, the variable arm of which is controlled by backwardly and forwardly operable motors or 'step-by-step selecting switches. With arrangements of this character trouble due to poor contact may be experienced both near the centre point of the potentiometer, since this will obviously get far more wear than the ends of its range, and also at the extreme ends of the range due to the small number of times such parts are brought into use.

To avoid these vdiliiculties it is proposed to adapt the known automatically variable resistance arrangement whereby two switches always operating in the same directionmove over a closed ring of equal resistances. According to the invention one of the switches is arranged to make contact with the ring at two points a fixed distance apart while the other switch is arranged to make contact with the ring at one point only so as to form a potentiometer arrangement.

With a simple arrangement of this type equal increments of movement of the switches produce equal changes of resistance and this does not lend itself conveniently to automatic gain control arrangements where it is desirable that the changes should be on a logarithmic rather than a linear basis. According to another feature of the invention therefore, two rings of resistances are provided in cascade, one switch being arranged to connect with each ring at two points a fixed distance apart while the other connects with each ring at one point only, the single connection of the second switch with the first ring extending to one of the connections of the first switch with the second ring so that a double potentiometer arrangement is formed whereby equal increments of movement of the switches give an approximately logarithmic change in output.

According to a further feature of the invention, in an automatic adjusting arrangement employing two power-driven switches operating always in the same direction to makeconnection with appropriate points on a ring [formed of a series of equal resistances, the range of adjustment is limited to a predetermined proportion of the ring and relay arrangements are provided for opencircuiting the portion of the ring outside the limits of adjustment.

The invention will be better understood from the following description of one method of carrying it into effect, reference being had to the accompanying drawings comprising Figs. l-l. Fig. 1 shows the essentials of the ring resistance potentiometer arrangement employing two unidirectional step-by-step switches which are respectively provided with wipers DGI, DGZ, and IGI, while Fig. 2 shows the perferred arrangement whereby two potentiometers of the type in question are connected in cascade to enable an approximately equal decibel change per step to be obtained. Fig. 3 shows graphically the decibel change per step obtained from the arrangement and also for comparison purposes shows the curve of an ideal arrangement which gives an equal decibel change per step. Figs. 4, 5 and 6 when arranged in the manner indicated in Fig. 4 show circuit details of the gain control arrangement according to the invention, and it will be understood that this employs two uni-directional reverse drive step-by-step 25-point selecting switches of the self-interrupted drive type in conjunction with seven controlling relays.

These switches are of the type commonly used and wel1 known in the automatic telephone art and a full description is given on pages 137-146 of "Telephony by Herbert and Procter, Part II, 1938 edition published by Pitman,

Fig. 7 shows schematically the general layout of the automatic gain control arrangement for which the circuit details are shown in Figs. 4, 5 and 6.

Referring now to Fig. 1 the input from the line is delivered to the terminals IE! and IE2 and extends via transformer TI to the ring resistance potentiometer PI which connects via transformer T2 with the output terminals CPI and 0P2. The levels of the switch on which wipers DGI and DGZ are provided are so wired that these wipers are in effect displaced by a constant number of steps and it will be seen that advancement of these wipers in the direction indicated serves to decrease the gain by reducing the voltage which is picked up by wiper IGI while movement of IGI in the direction indicated serves to increase the gain by increasing the voltage which is picked up. In order to break the continuity of the chain outside the portion over which wiper IGI operates, relay contacts are in eluded therein and at least one of these is always kept open in the unwanted position of the chain. Electrical means are furthermore provided to prevent wiper IGI passing beyond wiper DGZ and also to prevent the lagging wiper DG! passing beyond Wiper IGI as obviously no further gain or loss, as the case may be, can be obtained outside these two limits.

As previously mentioned, the arrangement shown in Fig. 1 sufiers from the limitation that each step of the potentiometer gives an equal voltage change so that in order to obtain an approximately equal decibel change per step the cascade potentiometer arrangement shown in Fig. 2 is adopted where they ratio of output to input varies according to a square law which approximates to the logarithmic decibel relationship theoretically required.

In Fig. 2 wipers DGIDG4 belong to one switch and wipers IGI and IG2 to theother, and the following typical resistance values are proposed to enable a load resistance of approximately 600 ohms to be maintained on transformer Tl.

Resistances YAYD serve as fixed or dummy steps which serve to control the efiective variable range of the. arrangement to give the values indicated on the curve of Fig. 3 where decibels are plotted vertically against potentiometer steps horizontally. The operating range under normal working conditions will be of the order of plus or minus 6113 from a pre-arranged nominal value and will, say, be between the points X and Y on the curve. From comparison between this part of the curve with the ideal characteristic curve of ldB per step (shown chain-dotted) it will be seen that the portion of the square-law curve which will be most often in use substantially corresponds to the ideal linear decibel curve.

Reviewing the efiect of the resistances YA-YD, resistances YA and YC ensure that the curve over the most used operating range under normal working conditions.

Detailed consideration will now be given to the arrangement shown in principle in 2, and from reference to Figs. 4-6, it will be seen that two 25-point uni-directional step-by-step switches are employed, each of which is provided with two sets of wipers 180 out of phase to enable an effective 50 points to be made available. The two switches which are designated DGS and IGS are provided with driving magnets DGM and IGM having interrupter contacts dgm and igm respectively. The wipers and banks on switch I DGS are designated DGSIDGSIO, while those and these-contacts are designated individually in small letters al, a2, a3 and a l. Relays DEG and IRG which are intended to be slow-to-operate and slow-to-release are shown with a cross in '50 resistances YEA-YEQ,

a small rectangle at one end to indicate this.

The ring resistance potentiometer Pl referred to in connection with Fig. 2 is formed from YFA-YFQ and YGA-YGP of 20 ohms each as shown in Fig. 4, while potentiometer P2 is formed from the 50 resistances YI-IA-YHQ, YIAYIQ and YJAYJ Q of 50 ohms each as shown in Fig. 5.

From reference to the wiring of the banks DGSL-DGSG, it will be seen that the wipers DGSl and DGS3, DGSZ and DGS4 are in client displaced from one another by 33 resistance steps, and similar remarks apply to wipers DGS'i and DGSQ, DGS8 and DGSlll. Wipers DGSI and DGS2 correspond to wipers DGI of Fig. 2, wipers DGS3 and DGSA to wiper DGZ of Fig. 2 and wipers IGSI and IGSZ to wiper IGI and similarly for potentiometer P2. It will be appreciated on examination that with the DGS switch occupying position 1 asshown, for maximum gain the IGS switch should occupy position 34 when all the input voltage delivered across the 33 resistances YEB-YFQ of potentiometer Pl will be picked up by wiper IGS4 and similarly in con nection with potentiometer P2. With the IGS switch in position 20 as shown, there will be a value of gain intermediate the minimum and maximum values obtainable. Movement of the IGS switch to position 34 serves to increase the gain to the maximum value. Movement of the DGS switch from position 1 would then serve to reduce the gain from the maximum value and when this switch reached position 34 corresponding to that occupied by the IGS switch, the gain would be a minium. With the DGS and IGS three positions, intermediate values of gain are obtained.

Considering now the actual equipment shown in detail in Figs. 4-6, and schematically in Fig. 7, the general arrangementwill first be dealt with. In Fig. '7, the signalling line is represented by I, and outgoing signals suitably modulated are extended thereto over the line 9. Incoming signals are extended by way of the directional low pass filter DLPF to the automatic gain control equipment I-IGC which corresponds to that shown in Figs. 4-6 and thence by way of the incoming common amplifier ICA to the line 8 extending tothe demcdulators. A branch from the line 8 extends by way of the selective filter SF, which is arranged to pass the pilot frequency amplifier and thence to the contact voltmeter D. This is arranged to apply earth to either lead H1 or lead ll according as the level of the pilot frequency is greater or less than the desired value and this initiates the operation of the automatic gain control equipment AGC to produce a suitable adjustment. It will be understood that the pilot frequency is applied to the distant end of the line 1 at a constant level. Assuming now that the amplitude of this frequency increases, earth will be applied by the contact voltmeter to the decrease gain lead It], Fig. 6. Relay DRG thereupon operates and at contacts dlrgl energises the magnet DGM of switch DGS, whereupon the magnet interrupter contacts dgm open and disconnect the circuit of relay DRG, which releases and de-energises the magnet after a short interval due to its slug. Due to the reverse drive action of the switch, the wipers now make one step from position 1 in which they are assumed to be positioned, the effect of which is to give a voltage reduction at the potentiometer of approximately a predetermined number of decibels, say one decibel.

This decrease of gain is effective on the received pilot frequency and if it is insufficient, the detecting contact voltmeter will maintain earth on lead l and relay DRG will reoperate when the magnet interrupter contacts dgm re-close. The wipers of switch DGS will therefore continue to advance step-by-step until earth is removed from lead ID.

The function of the slug on relay DRG is to enable the switch DGS to operate at a controlled speed which will give time for the alteration in signal level to affect the level measuring equipment so that the earth will be removed from the "decrease gain wire when the required correction has been made.

Similar remarks apply when it is necessary to increase the gain in which case earth will be applied to lead II and relay IRG and magnet IGM will then interact to cause switch IGS to advance its wipers and so increase the gain to effect the required correction.

With the DGS switch in position 1 as shown, it will be appreciated that the resistances between contact 35 and contact I tend to provide a parallel leak path which is not wanted. This path is, however, cut off by contacts a3 for potentiometer PI and a l for potentiometer P2 since relay A is operated when switch DGS is occupying position 1. Relay A on operating locks until one or more of the resistance steps between contacts 35 and l are required in the 33 steps of the potentiometer, that is to say, when switch DGS reaches position 18. At this-point relay B is operated and locks up and disconnects relay A, while at contacts 173 and b4 the now unwanted resistance portions between contacts I --|8 are disconnected. Similarly, when position 35 is reached by switch DGS, relay 0 is brought up to disconnect the then unwanted portions of the rings and it will thus be seen that the one of the contacts a3, D3 or 03 in the unwanted portion of the resistance ring PI is always kept open and similarly for the ring P2. It will be appreciated that this arrangement prevents the waste of power which would otherwise occur in the unwanted portions of the rings.

The limits of operation of the device. as previou'sly mentioned in connection with Fig. 1, are when the DGS and IGS switches occupy the same positions (minimum gain) Or when they are displaced by thirty-three positions (maximum gain).

In the former case relay DCO, Fig. 6, will be operated to cut off the decrease gain circuit at contacts deal and to connect an earth to the alarm lead 12 at contacts (1002. In the latter case relay ICO will be operated to disconnect the increase gain circuit and bring in an alarm by extending earth over lead l3.

In case it is desired manually to set the gain, for instance if the pilot frequency receiving device should fail, key COK is thrown whereupon contacts cokl and 00k! disconnect the signalling leads and by throwing key DGK or IGK the gain can be decreased or increased as required, after which the key will be restored. The required gain can be observed from the D. C. voltmeter VM, Fig. 5, connected across the output stage of the first stage potentiometer Pl, which obtains potential from a battery supply provided at the centre of transformer Tl, which supply also serves to maintain wet and uni-directional switch wiping contacts.

What I claim as new and desire to secure by Letters Patent is:

1. An electrical network adjusting arrangement comprising in combination a first switch provided with two brushes movable together in a circular path in one direction only, two banks of fixed con-' tancts arranged to be engaged respectively by said brushes, a second switch provided with a brush movable in a circular path in one direction only, a bank of fixed contactsarranged to be engaged by said brush, a plurality of equal resistances connected together to form a closed ring, connections between the junction point of each pair of adjacent resistances and the fixed contacts of the two banks of the first switch and the bank of the second switch such that the two brushes of said first switch engage junction points a predetermined number of resistances apart, an input circuit connected across the two brushes of said first switch, an output circuit connected across one of the brushes of said first switch and the brush of said second switch, means for operating the brushes of said first switch over their associated fixed contacts to produce adjustment in one sense between said input and output circuits and means for operating the brush of said second switch over its associated fixed contacts to produce adjustment in the opposite sense between said input and output circuits.

2. An electrical network adjusting arrangement comprising in combination a first switch provided with two pairs of brushes movable together in a circular path in one direction only, two pairs of banks of fixed contacts arranged to be engaged respectively by said two pairs of brushes, a second switch provided with two brushes movable together in a circular path in one direction only, two banks of fixed contacts arranged to be engaged respectively by the brushes of said second switch, a plurality of equal resistances connected together to form a first closed ring, a similar plurality of equal resistances connected to ether to form a second closed ring, connections between the junction point of each pair of adjacent resistances in said first ring and the fixed contacts of the first pair of banks of said first switch and the first bank of said second switch such that the first pair of brushes of said first switch engage junction points a predetermined number of resistances apart, connections between the junction point of each pair of adjacent resistances in said second ring and the fixed contacts of the second pair of banks of said first switch and the second bank of said second switch such that the second pair of brushes of said first switch engage'junction points a predetermined number of resistances apart, an input circuit connected. between said first pair of brushes of said first switch, a'contween the other of said second pair of brushes of said first switch and the other brush of said second switch, means for operating the brushes of said first switch over their associated fixed contacts to efiect adjustment in one sense between said input and output circuits and means for operating the brushes of said second switch over their associated fixed contacts to effect adjustment in the opposite sense between said input and output circuits.

3. An electrical network adjusting arrangement as claimed in claim 2 in which the input and output circuits comprise secondary and primary windings respectively of transformers forming part of a carrier current telecommunication system so that the adjusting arrangement serves to control the gain.

4. An electrical network adjusting arrangement as claimed in claim 2 in which fixed resistances included in the connections to the brushes of the first switch serve to determine minimum and maximum values for the adjusting operation.

5. An electrical network adjusting arrangement as claimed in claim 2 in which the means for operating the brushes of the two switches are controlled by opposite contacts of a contact voltmeter responsive to current transmitted through the adjusting network.

6. An electrical network adjusting arrangement comprising in combination a first switch provided with two brushes movable together in a circular path in one direction only, two banks of fixed contacts arranged to be engaged respectively by said brushes, a second switch provided with a brush movable in a circular path in one direction only, a bank of fixed contacts arranged to be engaged by said brush, a plurality of equal resistances connected together to form a closed ring, connections between the junction point of each pair of adjacent resistances and the fixed contacts of the two banks of said first switch and the bank of said second switch such that the two brushes of said first switch engage junction points a predetermined number of resistances apart, an input circuitconnected across the two brushes of said first switch, an output circuit connected between one brush of said first switch and the brush of said second switch, means for operating the brushes of said first switch over their associated fixed contacts to produce adjustment in one sense between said input and output circuits, means for operating the brush of said second switch over its associated fixed contacts to produce adjustment in the opposite sense between said input and output circuits, means for limiting the range of adjustment to a predetermined portion of thering of resistances and'means for open-circuiting the portion of the ring outside the range of adjustment. r

'7. An electrical network adjusting arrangement as claimed in claim 6 in which the means for open-'circuiting the portion of the ring outside the range of adjustmentcomprises a plurality of relays each controlling a predetermined portion Of the ring, means for operating each relay over a further movable brush of said first switch when said brush reaches the begin ning of the appropriate portion of the ring and locking circuits for each relay dependent on normally closed contacts carried by the relay asso ciated with the succeeding portion of the ring.

8. An electrical network adjusting arrangement comprising two switches each provided with a brush movable in a circular path in one direction only, two banks of fixed contacts arranged to be engaged by the brushes of the two switches respectively, a plurality of equal resistances connected together to form a closed ring, connections between the junction point of each pair of adjacent resistances and the fixed contacts of the banks of the two switches,an input circuit including the brush of said first switch,

- an output circuit including the brush of said second switch, a second movable brush on said first switch and connected to one pole of a current source, a bank of fixed contacts for said brush, a second movable brush on said second switch, a bank of fixed contacts for said brush, a first relayconnected to said second brush and to the other pole of said current source, a third movable brush on said second switch, a bank of fixed contacts for said brush, a second relay connected to said third brush and to the other pole of said current source, a first electromagnet arranged when intermittently energized to operate the brushes of said first switch to efiect adjustment in one sense between said input and output circuits, means for intermittently energizing said electromagnet, a second electromagnet arranged when intermittently energized to operate the brushes of said second switch to'effect adjustment in the opposite sense between said input and output circuits, means for intermittently energizing said second electromagnet, contacts carried by said first relay for disabling the energizing means for said first electromagnet, contacts carried by said second relay for disabling the energizing means for said second relay and interconnections between the banks of fixed contacts engaged by the second brush of said first switch and the second and third brushes of said second switch whereby said energizing means are disabled by the energization of said relays when opposite ends of a predetermined range of adjustment are reached.

ARTHUR CHARLES CORNER. 

